I run my two transfer lines through check valves then T them together and
through a fuel flow transducer driving a gage on the panel.  I also have an
automatic transfer system.  This way I can verify the pumps are transferring
fuel and a relay hasn't failed.  I cannot hear the pumps in flight.  The
system kicks on when the header drops to 5 gallons and turns off when it rises
to 9 gallons.

Ed de Chazal
N361DC